I am endebted to Maryx at FictionAlly for the suggestion that Voldemort means 'thief of death' and to Tom Lehrer for the song 'Poisoning Pigeons in the Park', which is alluded to.
All the rest belongs to JKR.
They do not understand.
Death is not the end but the summation. Death contains all of life within it. The spell merely sunders what is real from the cadaver.
It is this that He steals.
This that they consume to sustain the Dark Lord.
He was promised power, and power he has.
It is not his to use.
He is only a conduit. A gut for the Dark Lord's brain.
He hates it.
He did not betray his Lord. It was he who was betrayed. Loyalty that does not go both ways is not loyalty but something else.
And so he has another master whose loyalty is less certain. (For certainly Voldemort has none).
'Do what you must.'
And so he does what he must.
And he does what he can.
A word. A warning. Winged vermin impressed into the cause of light.
Severus Snape imperioing pigeons in the park.
He would laugh for he cannot cry.
Potter. On his own the fool stood no chance against him, and he has been robbed of vengeance for the decision was not his but another's.
Evans. He had thought that they were friends until she smiled.
A friendship that had brought her the attention and then the affection of Gryffindor's golden boy.
That had brought him contempt and pain.
He had not known then that sometimes people smile when they're afraid, and she had good reason to fear: not often did they tolerate defiance.
He has learned this long since.
And heDOES NOTWANT to do this.
'Get out of the way you foolish girl.' The child must die but the mother might yet be saved.
But she will not.
He sees that her hair is the colour of the setting sun and he braces himself against the brief pleasure and sudden pain of another's Death passing through his own soul.
He has seen them before, hung on the air before him like the sparkle of sunlight on water and drawn through him to darkness. He is but a conduit. Does the road to the scaffold grieve?
She has offered her life for her child's and, without husband or child, surely this is kindness?
He is unaware of having spoken the words but now he realises that She is not merely the reflection but the blazing noon sun itself and She must pass through him and he is afraid, sure that She cannot do so without destroying him utterly. It is his own magic struggling for his very existence that is pulling the house down around them, and unthinkingly he protects the child, crawling blindly away from him. He can feel the hunger of the Dark Lord opening himself to swallow Her and he cannot. It is the darkness that is engulfed.
Through long hours he lies pinned and broken under rubble while the child is rescued and events unfold and in his mouth is a taste like the scent of summer meadows. In the quiet morning Dumbledore comes for his spy.
When he sleeps he dreams of fields of asphodel.
He has applied for the Dark Arts position again.
He knows that he will not get it, but in the dungeons he sees too much. In a massive vault of masonry and native granite he comes upon a bier with a simple box of dark wood on it. He is unsurprised to see that the small silver plaque bears his name. She lays a wreath of white flowers over it.
She says nothing but his own voice speaks to him continually. Words, words and more words. He spins elegant silken phrases, weaving the tale of his own existence amongst the lives of others. He hangs in a web of words slung across the abyss. He drinks, he eats, he breathes but, even so, some of the children believe that he's a vampire. If it were that simple he would have an answer, but his body is the only tool they have. She is beautiful and he is so very afraid.
He watches the child. This prideful, stubborn, wilfully ignorant boy cannot defeat the Dark Lord but, even so, he guards him.
'I'm trying to decide which spell to use on Malfoy, sir.' Snape stares at the boy. She is standing before him pleading, demanding and he would give her anything but that he does not know how.
At the Headmaster's request he has arrived to escort Crabbe, Goyle and Malfoy home from Saint Mungo's and in the corridor outside the ward, for the first time, he sees that she is crying.